1. Field of the Invention
The present invention relates to a subscriber radio access system. More particularly, it relates to a subscriber radio access system on Ethernet for linking a radio base station and subscriber terminals through a radio transmission line.
2. Description of the Related Art
In recent years, it has been increased to have chances for using Internet in both of companies and each home because ISDN lines are now widespread among the general public, and providers appear to provide services for Internet with low costs. However, when considering infrastructures for communication in each home, only a telephone line is installed, and therefore, high-speed communications can not be facilitated in each home under the existing condition.
On the other hand, a conception of Fiber to the Home (FTTH) is held among enterprises related to a communication industry. However, a huge infrastructure investment is required, and therefore, it is considered that it will be in about 2010 to completely install the FTTH in each home. In this situation, a subscriber radio access system in which high speed transmission channels can be provided to each home by employing radio transmission channels has been expected much in recent years.
In here, a Point-multi point system for linking one radio base station and a plurality of subscriber terminals is generally employed as the subscriber radio access system. When considering the entire configuration, a cellular zone structure, as used in a mobile telephone system and a cellular phone system, is employed.
When forming a Point to multi-point connection by the use of radio communication, it is general to employ a Time Division Multiple Access (TDMA) system to transmit and receive communication data between the base station and the subscriber terminals with a predetermined frame signal.
In the case of using a TDMA system, it is possible to facilitate communications stably without collision of signals while each subscriber facilitates communications. However, when employing data communication, such as Internet communication in each home, there is much time when the subscriber does not facilitate communications. In this case, each subscriber terminal may keep a certain time interval in a predetermined frame so that transmission speed becomes constant in each subscriber terminal even if a few subscribers facilitate communications.
Accordingly, it is no use because there are many unemployed signal periods on the whole signal transmission. Further, even if the TDMA system can be realized, as it is not standardized currently, each subscriber terminal should be recognized as a special one, thereby increasing the cost and taking much time to expand the system.
To cover the shortage of the above-described problem, a wired LAN is standardized. On Ethernet, which is a bus type network for short distance transmission, i.e., a lower layer protocol in a stack of Transmission Control protocol/Internet protocol (TCP/IP), employed as a typical communication method on LAN, it is effective to employ a method for transmitting and receiving a packet only when facilitating communications according to Carrier Sense Multiple Access (CSMA) protocol.
However, as Ethernet follows a standard of the wired LAN, there is a limited transmission distance to make it possible to detect data collision even if the packet format has a minimum length. For example, in the case of 10 BaseT, i.e., a data communication speed of 10 Mbps employing a twist cable, the maximum length of the cable is limited to 100 m. Therefore, in the case where a radio transmission link section is longer than the limited length, it becomes impossible to detect data collision, thereby not facilitating the data communication.